The Integrated Services Digital Network (ISDN) is a digital switching and network architecture which provides end-to-end digital signal connectivity and uses an out-of-band signaling capability. The end-to-end digital connectivity allows signals to enter the network in digital form, be transported in digital form, and be delivered to the user in digital form. This feature of ISDN can, in some circumstances, eliminate the need for a modem which converts the digital signals into analog form for transmission over an analog telephone network. The out-of-band signaling feature allows the signaling information required to administer a communications function (callsetup and take down, call monitoring, call waiting tones, etc.) to be transmitted to the central office switch by a separate signaling channel. This out-of-band signaling can therefore eliminate the need to use channel space in a talk path for administrative purposes. In an ISDN network, all signals, whether voice, data, or administrative, are transferred in digital form. Two types of ISDN communications channels are provided to the ISDN user. These are known as B channels and D channels. The D channel contains signaling and/or data information and the B channel contains voice or data information. The B and D channels are multiplexed to form a single bit stream containing user information (voice and data) and user control (signaling). Both B and D channels are bi-directional.
At present, the telephone companies provide ISDN facilities only in limited areas of a few major cities. A person wishing to test and/or evaluate ISDN hardware and/or software must therefore take the hardware and/or software to an area in a city in which the local telephone company provides ISDN services. This procedure is costly, time consuming and ineffective since, each time the hardware and/or software is modified another trip must be made to evaluate whether the modification has corrected the previously existing problem and to ascertain that new problems have not been created.
Therefore, there is a need for a method and an apparatus which will allow a person developing ISDN compatible equipment to gain access to an ISDN network without having to travel to a city where ISDN services are available.
The lack of ISDN capability in most areas of the country also presents a problem for a potential purchaser of an ISDN-compatible device. The ISDN-compatible device may have all the features desired and/or required by the potential purchaser except for one: the device cannot operate over a conventional analog telephone line. If the potential purchaser is not in an area where ISDN services are provided by the local telephone company the potential purchaser will not be able to use the desired device.
Therefore, there is a need for a method and an apparatus which will allow an ISDN compatible device to communicate with another ISDN device over a standard analog telephone network.